obstack: fix compiler warning

MS Visual C suggests that the construct

	condition ? (int) i : (ptrdiff_t) d

is incorrect. Let's fix this by casting to ptrdiff_t also for the
positive arm of the conditional.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/obstack.h

@@ -496,7 +496,7 @@ __extension__								\
 ( (h)->temp.tempint = (char *) (obj) - (char *) (h)->chunk,		\
   ((((h)->temp.tempint > 0						\
     && (h)->temp.tempint < (h)->chunk_limit - (char *) (h)->chunk))	\
-   ? (int) ((h)->next_free = (h)->object_base				\
+   ? (ptrdiff_t) ((h)->next_free = (h)->object_base				\
 	    = (h)->temp.tempint + (char *) (h)->chunk)			\
    : (((obstack_free) ((h), (h)->temp.tempint + (char *) (h)->chunk), 0), 0)))